With digitalization of electronic apparatuses, there is a demand for capacitors having a small size, a large capacitance, and a small equivalent series resistance (hereinafter, abbreviated as “ESR”) in a high frequency region to be used at a power supply output side of circuits such as, for example, a smoothing circuit and a control circuit. As such capacitors, electrolytic capacitors using a fluid electrolyte typically such as an electrolyte solution have been used. Furthermore, recently, solid electrolytic capacitors using a solid electrolyte such as manganese dioxide, a TCNQ complex salt, or an electroconductive polymer such as polypyrrole, polythiophene, and polyaniline have been used.
The solid electrolytic capacitor is excellent in that it has a particularly low ESR as compared with a liquid-type electrolytic capacitor. However, the solid electrolytic capacitor is poor in repairing a defective part in anodic oxide film as a dielectric body. Therefore, leakage current may increase, and, in the worst case, a short circuit may occur.
Meanwhile, particularly, in recent AV apparatuses and automobile electrical equipment, high reliability has been increasingly demanded. Therefore, also in solid electrolytic capacitors, low leakage current and a short-circuit resistance property, in addition to performance such as having a small size, a large capacitance, and a low ESR, are being demanded. In order to meet such demands, a so-called hybrid-type electrolytic capacitor using an electrolyte solution, as material for an electrolyte, that is excellent in repairing a defective part in an anodic oxide film that is a dielectric body in addition to a solid electrolyte such as an electroconductive polymer has been proposed.
FIG. 4 is a sectional view showing a configuration of a hybrid-type electrolytic capacitor (having wound-type capacitor element) as an example of conventional electrolytic capacitor. FIG. 5 is a development perspective view of a capacitor element of the hybrid-type electrolytic capacitor. As shown in FIG. 4, the hybrid-type electrolytic capacitor includes capacitor element 2 as a function element, a pair of lead wires 1A and 1B, and outer package 5. One end portion of each of lead wires 1A and 1B is connected to capacitor element 2. Outer package 5 encloses capacitor element 2 and an electrolyte solution (not shown) in such a manner that the other end portion of each of lead wires 1A and 1B is led to the outside.
Outer package 5 includes bottomed cylindrical case 3 and seal member 4. Case 3 accommodates capacitor element 2 impregnated with the electrolyte solution. Seal member 4 is provided with through holes 4A and 4B through which lead wires 1A and 1B are inserted, respectively. Seal member 4 is compressed by drawing processing part 3A provided on the outer peripheral surface of case 3 so as to seal an opening of case 3. Seal member 4 is made of rubber packing.
As shown in FIG. 5, capacitor element 2 includes anode foil 2A, cathode foil 2B, and separator 2C. Anode foil 2A is formed by roughing a foil made of a valve metal such as aluminum by etching, and forming an anodic oxide film (not shown) as a dielectric body thereon by anodization. Cathode foil 2B is formed of a valve metal such as aluminum. Separator 2C is disposed between anode foil 2A and cathode foil 2B. In this state, anode foil 2A, cathode foil 2B and separator 2C are laminated and wound so as to form capacitor element 2. A solid electrolyte layer (not shown) made of an electroconductive polymer such as polythiophene is formed between anode foil 2A and cathode foil 2B.
One end portion of lead wire 1A is connected to node foil 2A, and one end portion of lead wire 1B is connected to cathode foil 2B. The other end portions thereof are led out from one end surface of capacitor element 2.
The electrolyte solution includes a solvent, a solute, and additives, and it is based on an electrolyte solution that has been used in a conventional liquid-type electrolytic capacitor using only a liquid electrolyte. The liquid-type electrolytic capacitors are roughly classified into electrolytic capacitors having a low withstand voltage in which a rated voltage is not greater than 100 W.V. and having a low ESR, and electrolytic capacitors having a high withstand voltage in which a rated voltage is, for example, 250 W.V., 350 W.V., and 400 W.V. Mainly, the former electrolytic capacitors are used in a smoothing circuit and a control circuit at the power supply output side, and the latter electrolytic capacitors are used in a smoothing circuit at a power supply input side. These are largely different from each other in various properties of an electrolyte solution to be used in each electrolytic capacitor because roles in the circuit and material compositions are different from each other. Therefore, these electrolyte solutions cannot be used compatibly.
On the other hand, the hybrid-type electrolytic capacitor is used in a smoothing circuit and a control circuit at the power supply output side because it has an ESR as low as that of a solid-type electrolytic capacitor and has a limitation with respect to a withstand voltage. Therefore, a conventional hybrid-type electrolytic capacitor employs an electrolyte solution having high electric conductivity and an excellent low-temperature characteristic, which is applicable for conventional liquid-type electrolytic capacitors. Specific examples of the electrolyte solution is an electrolyte solution including γ-butyrolactone, ethylene glycol or the like as a main solvent, and amidine phthalate, tetramethylammonium phthalate, ammonium adipate, triethylamine phthalate or the like as a solute.
In a conventional hybrid-type electrolytic capacitor configured as mentioned above, an electrolyte solution enters into ores in the solid electrolyte layer of an electroconductive polymer formed in capacitor element 2, and thus, a contact state between a dielectric oxide film and the electrolyte is improved. Therefore, the capacitance is increased, the ESR is lowered, repairing of a defective part in the dielectric oxide film is promoted by the effect of the electrolyte solution, and thus leakage current is reduced. Such an electrolytic capacitor is disclosed in, for example, PTLs 1 and 2.
Electrolytic capacitors used in AV apparatuses and automobile electrical equipment require high reliability over a long period of time. Such electrolytic capacitors are used under a harsh environment at high temperatures, for example, at a maximum working temperature of 85° C. to 150° C. for a long time. Meanwhile, a conventional hybrid-type electrolytic capacitor has a configuration in which an opening of a case accommodating a capacitor element and an electrolyte solution is sealed by sealing material such as rubber and epoxy resin, and therefore lifetime design becomes important.
However, a solvent of the electrolyte solution used in a conventional hybrid-type electrolytic capacitor is a volatile organic solvent such as γ-butyrolactone, ethylene glycol, and sulfolane. Therefore, when the electrolytic capacitor is exposed to a high temperature, the solvent gradually penetrates into a gap between the seal member and the case, a gap between the seal member and the lead wires, or the seal member itself, and gradually vaporizes and volatilizes. In general, an electrolytic capacitor is designed to have a guaranteed lifetime such that a range in which stable properties can be maintained with variation of the physical properties of material to be used or manufacturing conditions taken into consideration. However, if the electrolytic capacitor is used for a long time beyond the guaranteed lifetime, a solvent in the electrolyte solution is finally lost. Therefore, a function of self-repairing a defective part in the dielectric oxide film is lost.
In a conventional liquid-type electrolytic capacitor using only liquid electrolyte, even if the solvent of the electrolyte solution is lost, since the dielectric oxide film and the cathode foil are insulated from each other by the separator, electrolytic capacitor is only to be in an open mode and not in a short circuit.
On the other hand, in the hybrid-type electrolytic capacitor, even if the solvent of the electrolyte solution is lost, an electroconductive solid electrolyte layer remains between the dielectric oxide film and the cathode foil. Therefore, when an effect of the electrolyte solution is lost, an increase in leakage current is caused. As a result, in a worst case, a short circuit occurs.